Cold cathode ionization gauges are known in the art. Typically, such gauges are employed for making vacuum measurements, i.e., measurements at very low pressure (e.g., 10.sup.-1 to 10.sup.-7 torr). Three commonly known cold cathode ionization gauges include normal (noninverted) magnetron type gauges, inverted magnetron type gauges, and Philips (or Penning) gauges. All of these types of gauges have a pair of electrodes in an evacuated non-magnetic envelope. The envelope has an opening which communicates with the vacuum to be measured. A high D.C. voltage is applied between the electrodes for causing a discharge current to flow therebetween. A magnetic field is applied along the axis of the electrodes in order to help maintain the discharge current at an equilibrium value which is a function of pressure. The level of vacuum of an enclosure connected to such a gauge can thus be obtained by measuring the amount of the discharge current within the gauge.
In contrast with cold cathode gauges, hot filament gauges emit electrons by heating a filament. The electrons are attracted toward a positively charged grid electrode. Collisions of electrons with gas molecules produce ions, which are then attracted to a negatively charged electrode. The current measured at this electrode is directly proportional to the pressure or gas density.
One form of prior art normal (noninverted) magnetron type and inverted magnetron type gauges include a pair of concentric cylinder-shaped electrodes, typically consisting of an anode and a cathode, spaced apart and insulated from one another. The two cylinders can thus be defined as inner and outer cylinders. A high voltage is applied between these cylindrical electrodes. The high voltage accelerates electrons traveling from the negative to the positively charged cylindrical electrode. A magnet placed around the outer cylinder provides a magnetic field which is parallel to the axis of the cylinders. This magnetic field causes the electrons to orbit in the space between the two electrodes instead of going directly to the positively charged electrode. This long orbiting path increases the probability of collision of the electrons with residual gas molecules between the inner and outer cylinders, thus improving the production of positive gas ions. The flow of these ions to the negatively charged cylinder can be detected as a current flow. This current flow is proportional to the number of molecules of gas. Since the volume of gas can be determined, the current flow can be used to calculate the gas pressure.
U.S. Pat. No. 4,270,091 discloses a Penning type vacuum pressure gauge. FIGS. 3, 5 and 7 all disclose embodiments employing a cathode, anode and two magnets. In FIG. 3, anode 21 is a stainless steel cylindrical tube. Cathode 20 is a sheet material formed into a U-shaped member. The parallel end faces of the U-shaped cathode baffle the open ends of the cylindrical anode, as depicted in FIG. 4. A magnetic field is applied along the axis of the anode between a pair of permanent magnets 49 attached to a magnetic yoke 36. The yoke completes the magnetic circuit between the two magnets. The two magnets are arranged around the cylindrical tube so that their magnetic fields add rather than oppose. In FIG. 5, two cylindrical electrodes 61 are spaced apart on a common axis to form cathodes. A cylindrical anode 62 of smaller or larger radius is coaxially disposed intermediate the cathodes. Annular permanent magnets 64 and 65 apply a magnetic field parallel to the axis. The magnets 64 and 65 are located such that their magnetic fields add, rather than oppose. In FIG. 7, a cathode such as filament 82 injects electrons by acceleration through a gridded anode 83 into a magnetic confinement region formed by two annular magnetic sources spaced apart on a common axis, as for example solenoids 84 and 85. The confined electrons execute complex trajectories principally characterized by helical motion (indicated by line e) caused by magnetic lines of force having the shape indicated generally at 87, and these electrons are reflected from regions of more intense magnetic field. The magnetic field creating elements, solenoids 84 and 85, are located so that their magnetic fields add, rather than oppose.
U.S. Pat. No. 3,435,334 discloses an ionization vacuum gauge wherein electrons emitted from floating cathode 12 and are attracted to the adjacent surface of grid cage 13, travelling radially from the cathode to the grid. This patent discloses an alternative embodiment wherein a D.C. magnetic field may be established along the electron path between the cathode and grid. An exemplary embodiment of a structure to establish such a magnetic field uses bar magnets 16 and 17 located on either side of the cathode/grid structure. The magnets reduce the interception of electrons attracted to the grid from the cathode and thereby increase the electron density within the interior of the grid cage.
U.S. Pat. No. 3,872,377 discloses a cold cathode ionization gauge for increasing the accuracy of vacuum measurements by employing a high voltage pulse generator for supplying a high voltage pulse of limited duration between a magnetic field generating means for applying a magnetic field along the axis of electrodes disposed therein. FIGS. 1, 5 and 6 of this patent depict a normal (noninverted) magnetron type gauge, inverted magnetron type gauge and Philips gauge, respectively. All of these embodiments employ a very heavy single permanent magnet along the axis of the electrodes. FIG. 7 of this patent depicts an alternative embodiment wherein the permanent magnet is replaced by an electric magnet which produces a pulsating magnetic field in synchronization with the application of the pulsating high voltage for causing a discharge within the vacuum gauge. This patent notes that one advantage of the electric magnet is that it is of light weight compared to a permanent magnet.
U.S. Pat. No. 4,000,457 discloses in FIG. 1 a cold cathode ionization gauge tube 60 comprising a central cathode 61, a coaxial anode 62 and a single magnet 63. The operating range of this gauge is extended by employing circuit elements attached to its tube, rather than by modifying aspects of the magnetic field.
U.S. Pat. No. 4,967,157 discloses a cold cathode discharge vacuum gauge which employs a conventional-type tube for pressure measurements. FIGS. 5 and 6 of this patent depicts a cold cathode ionization tube 12 of the inverted magnetron type having separate feedthroughs for anode 14 and cathode 16. The required magnet is not shown. The operating range of this gauge is also extended by employing circuit elements attached to its tube, rather than by modifying aspects of the magnetic field.
U.S. Pat. No. 3,796,917 discloses an ionizer for ionizing residual gas molecules in a vacuum spaces, and describes different techniques to enhance the magnetic field axial to the space between a cathode and anode. However, in all of the embodiments disclosed in this patent, only a single magnet is employed.
U.S. Pat. Nos. 2,884,550, 3,171,081, 3,378,712 and 3,505,554 all disclose tube-shaped ionization gauges employing a cylindrical anode, a filamentary cathode (rod) which runs centrally through the tube, and a magnet around the periphery of the tube. In all of these four patents, only a single magnet is employed.
The above-described gauges all use magnetic fields, because the magnetic field created within the ionization tubes allows for a long electron trajectory which, in turn, results in a high ionization efficiency and the ability to operate in a low vacuum. However, the need to employ strong magnets to create a useful magnetic field creates numerous disadvantages. One such disadvantage is that the magnets create strong external magnetic fields which interfere with neighboring electrical and electronic devices.
Accordingly, there is still a need for a cold cathode ionization gauge that has a magnetic field configuration which minimizes external magnetic flux. There is also a need for a gauge that can be ignited in a very high vacuum, thereby extending the operating range of such gauges. There is further a need for achieving this goal through a design that is inexpensive and simple to fabricate. The present invention fills those needs.